The first practical automated mass soldering technique developed for soldering printed wiring boards is wave soldering. In such a system, conveyors are utilized to either direct feed the printed circuit boards or a double chain conveyor carries board carriers or pallets. The printed circuit boards have a deposit of flux placed thereon by any one of three techniques, namely foam fluxers, wave fluxers or spray fluxers. Foam fluxers are utilized for single-sided and most double-sided printed circuit boards and normally there is a control deposit of flux on the printed circuit board
Thereafter, the board is preheated and solder material is deposited utilizing any number of the different types of solder pots. After being conveyed through the solder unit, the printed circuit boards are cleaned and conveyed into a point where they can be removed from the soldering conveyor. The present invention is directed to the problems of foam fluxers utilized in such systems.
Over the years, many soldering fluxes have been developed for use in automated printed wiring board soldering. One class which is widely used is composed of liquid rosin fluxes. In foam application, finely dispersed air is passed through the flux causing it to foam. Then the printed circuit boards are passed over the foam to apply the uniform deposit of flux on the board. The success of this type of mass soldering depends upon having the materials and process variables under control at all times.
One process variable which creates problems is the composition of the foam fluxes. All methods of applying fluxes result in evaporation of the flux solvent through use. However, foam fluxing increases this problem because of the use of gas which causes greater evaporation of the solvent in the flux solution. Evaporation of the flux solvent results in a progressive increase in the specific gravity of the flux. As time goes on, the relative solid content of the flux increases with the result that more solids stay on the board.
Generally, the preheaters are set to flash off the flux solvent and preheat the printed wiring board to start the flux action and reduce thermal shock. As the flux film gets thicker, removal of flux solvent takes more of the heat, leaving less for starting flux action and less for reducing thermal shock. The result, of course, is a variation in the process for which the machine can't compensate. Another result of uncontrolled solvent evaporation is that flux consumption increases because drag-out on the boards is greater. Excessive flux also means increased cleaning costs. It, therefore, would appear obvious that it would pay to control flux specific gravity during its use.
Flux specific gravity can be controlled by measuring specific gravity and adjusting by adding thinner and/or solvent until the gravity is brought back to the correct value. This can be an effective means of control if done frequently enough and accurately enough. In practice, however, checks and adjustments tend to be too infrequent to maintain the control which is desirable. Solvent control is even more important in the case of foam fluxing than it is in the case of brush or wave fluxing. First, the evaporation rate is considerably greater when the gas is bubbled through a flux and, secondly the ratio of active ingredients and/or solids to solvent is critical in maintaining the foam-properties of the flux.